Abstract:
The grass weed Eleusine indica is listed as one of the most problematic weeds species
in the world. It is characterized by vigorous growth and an extended root system that
contributes to making this weed highly competitive. The discovery of glyphosateresistant
(GR) biotypes across the world, which currently places E. indica in the top five
in the list of GR weeds, has only exacerbated the problem status of the weed.
Glyphosate is still regarded as the most important herbicide and has become the most
historically successful herbicide throughout the world. Eleusine indica populations that
proved to be resistant to glyphosate have been identified under various cropping
situations and are now amongst the most widely distributed herbicide resistant species,
causing great economic impact worldwide. This has triggered interest on the biology of
this species and its response to glyphosate in order to improve its control. Several
studies have been conducted on this species across the world but no studies have been
conducted in South Africa. Therefore, there is a need for a better understanding of the
biology of the weed and information on effective control of this cosmopolitan weed in
South Africa as well as globally. The objectives of the present study were to determine
the effects of temperature and light on seed germination of E. indicaas well as to
investigate the response of E. indica to glyphosate. Growth chamber and glasshouse
experiments were conducted on a population of Eleusine indica obtained from Hatfield
experimental farm of the University of Pretoria (location: 25°45'7.08"S, 28°15'33.12"E).
All experiments were conducted at phytotron facilities on the experimental farm. In the
growth chamber, experiments were conducted to assess the germination behaviour of
E. indica under different temperature and light regimes with different germination media.
The lowest germination rate was recorded in H2O medium, under light/dark conditions
(82.42%). Seeds germinated best in KNO3 medium regardless of alternating
temperature regimes or light conditions. The highest final germination was recorded in
KNO3 medium in the dark (99.17%). Germination of E. indica was high (>80%) at all
three alternating day/night temperature regimes and light interactions. The response of
E. indica to glyphosate was investigated in a growth chamber with a rapid in vitro
experiment and in the glasshouse by means of a dose-response method. Two in vitro methods were tested with un-germinated and pre-germinated seeds placed on filter
papers in Petri dishes to which solutions with glyphosate concentrations of 0 and 900 g
a.e. ha-1 were added. After seven days, root and shoot lengths of the seedlings were
measured. The shoot and root development of both the un-germinated and pregerminated
seeds were significantly reduced by glyphosate. In the glasshouse, a doseresponse
experiment was conducted to determine the response of E. indica to a range
of glyphosate dosages. Glyphosate dosage rates were 0, 0.25x, 0.5x, 1x, 2x and 4x the
registered label recommended rate of Roundup Turbo® (1x = 900 g a.e. ha-1). Biomass
was measured 28 days after treatment. All glyphosate doses examined in the doseresponse
experiment significantly reduced the biomass of E. indica. The findings of both
these experiments reveal that the E. indica population examined in this study is
sensitive to glyphosate. Finally, the effect of growth stage on the response of E. indica
to glyphosate was investigated in the glasshouse. Plants were treated with glyphosate
at the field recommended dose of Roundup Turbo®, 900 g a.e. ha-1 at four different
growth stages: 21, 35, 49, and 63 days after emergence (DAE). Control treatments
were maintained for comparison for each growth stage. Biomass was measured 28
days after treatment. While plant biomass was significantly reduced by glyphosate,
older plants (63 DAE) were visually less affected by glyphosate. Despite glyphosate
treatment, these plants continued to grow and produce seeds. Therefore, growth stage
had an effect on the response of E. indica to glyphosate. The results of this study
demonstrate the importance of growth stage on glyphosate application even where
plants are sensitive to the herbicide. Growers should therefore consider and be aware
of the importance of timing glyphosate application. There is a need to intensify this
study to examine populations from various geographic locations. Germination
characteristics and weed response methods for weeds from different locations might
differ as a result of different climatic conditions and weed control practices. This
information is essential for improving methods currently available for control of this
weed in the country, and perhaps even in other parts of the world.